Sol-forming cerium material

ABSTRACT

A novel cerium (IV) material, well adopted for the production of aqueous colloidal sols therefrom, has the general formula (I): 
     
         Ce(M).sub.t (CH.sub.3 --CH.sub.2 ].sub.n COO.sup.-).sub.x (OH.sup.-).sub.y 
    
      (NO 3   - ) z                                     (I) 
     whereein M is an alkali metal or a quaternary ammonium radical; t ranges from 0.1 to 0.3; n is 0 or 1; x ranges from 0.1 to 0.7; y is a number such that y=4+t-x-z; and z ranges from 0.3 to 0.6.

CROSS-REFERENCE TO RELATED APPLICATIONS

Copending Applications Ser. Nos. 876,449, 876,635 and 876,681, all nowabandoned, all filed concurrently herewith and all assigned to theassignee hereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel compound of cerium (IV) and toa process for the preparation thereof. This invention more especiallyrelates to a novel compound of cerium (IV) that is readily dispersed inwater.

2. Description of the Prior Art

It is known to this art, from the Kirk-Othmer Encyclopedia of ChemicalTechnology, 2nd edition, Vol. 4, p. 850, that a hydrated ceric dioxideof the formula CeO₂.xH₂ O may be prepared, wherein x is a number rangingfrom 0.5 to 2, in the form of a gelatinous precipitate by the additionof sodium or ammonium to solutions of ceric salts.

It has also been proposed to this art, according to French Patent No.2,482,075, to prepare a cerium (IV) compound which is dispersible inwater by dispersing an essentially dry hydrate of cerium (IV) oxide inan aqueous medium, the same having been subjected to a heat treatment ata temperature ranging from 200° C. to 450° C., in the presence of adisintegrating agent, in particular nitric acid. Heating in the presenceof a disintegrating agent effects the disintegration of the aggregatedcrystallites into hydrated cerium (IV) oxide, thereby producing adispersible cerium compound.

It is noted in this '075 patent that the preparation of a hydrate ofcerium (IV) oxide may be carried out by precipitation beginning with acerium salt; thus, for example, a high purity cerous carbonate may bedissolved in a nitric acid or hydrochloric acid solution to obtain aneutral solution of cerous nitrate or chloride, which is then oxidizedwith NH₄ OH/H₂ O₂ to obtain the hydrate of cerium (IV) oxide.

SUMMARY OF THE INVENTION

A major object of the present invention is the provision of a novelcompound of cerium (IV) that is easily dispersed in water and which isprepared directly from a cerium (IV) salt without the need for aprecipitation stage and separation from ceric hydroxide.

The novel cerium (IV) compound according to this invention has thefollowing general formula (I):

    Ce (M).sub.t (CH.sub.3 --CH.sub.2 ].sub.n COO.sup.-).sub.x (OH.sup.-).sub.y (NO.sub.3.sup.-).sub.z

wherein M is an alkali metal or a quaternary ammonium radical; t rangesfrom 0.1 to 0.3; n is 0 or 1; z ranges from 0.1 to 0.7;y is a numbersuch that: y=4+t-x-z; and z ranges from 0.3 to 0.6.

Analysis of this novel compound by X-ray diffraction evidenced that itis a poorly crystallized product having a CeO₂ type structure.

The present invention also features a process for the preparation of thenovel compound (I) by (i) mixing an aqueous solution of a cerium (IV)salt with acetic acid or propionic acid; (ii) reacting the resultingmixture with a base; (iii) heat treating the mixture of reaction; (iv)separating the precipitate which is thus produced; and thence (v) dryingsaid separated precipitate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

More particularly according to the present invention, in the first stageof the subject process, an aqueous solution of a cerium (IV) salt andacetic or propionic acid, which themselves may be in aqueous solution,is utilized.

The starting solution of the cerium salt according to the inventionadvantageously is an aqueous solution of ceric nitrate, or an aqueoussolution of ceri-ammonium nitrate. The solution may contain, withoutcausing difficulties, cerium in the cerous state, but it is desirablethat it contain at least 85% of cerium (IV).

The solution of the cerium salt is selected such that it will notcontain impurities which could be transferred into the final product. Inparticular, it is preferable that it does not contain covalent anionswhich are coagulating in nature, such as sulfates, and the like.However, small amounts may be tolerated. For example, said anions mayconstitute up to 5% by weight of the cerium salt, expressed as CeO₂.

The concentration of the solution of the cerium salt is not criticalaccording to this invention. If it is expressed in terms of cerium (IV)values, it may advantageously vary from 0.1 to 2 moles per liter. It maybe important for the productivity of the equipment employed to use aconcentrated solution of the salt of cerium (IV); a concentration offrom 1 to 2 moles per liter is preferred.

The aqueous solution of the cerium (IV) salt has a certain initialacidity and may have a normality ranging from 0.1 N to 4 N. Theconcentration in H⁺ ions is not critical. It is desirable that it rangefrom 0.1 N to 1 N.

The solution of ceric nitrate obtained by the electrolytic oxidation ofa solution of cerous nitrate and described in published FrenchApplication No. 2,570,087 (No. 84/13641) is one starting material ofchoice.

The acetic acid or propionic acid is selected such as to be free ofimpurities. They may be used diluted, for example 1 N, or concentrated.Preferably, concentrated commercial acetic or propionic acid is used.

Acetic acid is preferred over propionic acid.

The proportion of acetic acid or propionic acid to be used with respectto that of the cerium (IV) salt is not critical. The molar ratio betweenthe acid and the cerium (IV) salt, expressed as CeO₂, advantageouslyranges from 0.01 to 15 and preferably from 0.5 to 2.

According to the process of the invention, the mixture is prepared byagitating the aqueous solution of the cerium (IV) salt and the acetic orpropionic acid, at a temperature ranging from room temperature to 80° C.It is not advisable to exceed 80° C., in order to prevent thedecomposition of the acid.

In the second stage of the process, a base is added to the resultingmixture.

The basic solution used according to the invention may be, inparticular, an aqueous solution of ammonium, sodium or potassiumhydroxide. Gaseous ammonia may also be used. Consistent herewith, anammonium solution is the preferred.

The normality of the basic solution is not critical according to theinvention; it may vary over a wide range, for example, from 0.1 to 11 N,but it is preferable from an economic point of view to use aconcentration ranging from 5 to 11 N.

The proportion of the basic solution to the solution of the cerium (IV)salt should be such that the molar ratio of OH⁻ /Ce (IV) is greater thanor equal to 1 and less than or equal to 4, with OH representing thenumber of moles of OH⁻ introduced by the addition of the base and Ce(IV) representing the number of moles of Ce (IV) present in the reactionmedium.

Preferably, a molar ratio of OH⁻ /Ce (IV) greater than or equal to 1.5and less than or equal to 3.5 is selected.

The reaction between the mixture obtained in the first stage and thebase added in the amounts defined above, is advantageously carried outat a temperature ranging from 0° C. to 60° C., but preferably at roomtemperature (most typically 15° to 25° C.).

The duration of the reaction is also not critical and depends upon thecapacity of the apparatus employed. It may vary from 1 second to 20hours.

The mixture of the aforementioned reagents may be carried out accordingto any one of a number of different embodiments. For example, admixturesof the aqueous solution of the cerium (IV) salt containing the acetic orpropionic acid with the basic solution may be carried out simultaneouslyunder agitation, or the base may be added continuously, or in a singlebatch, to the aqueous solution of the cerium (IV) salt containing theacetic or propionic acid.

In a third stage, the reaction mixture is heat treated at a temperatureranging from 50° C. to 100° C., and preferably from 70° C. to 90° C.

The reaction mixture may be immediately subjected to the selectedtemperature, or the temperature thereof may be gradually increased tothe desired value.

The conditions of the heat treatment are not critical; the treatment maybe carried out in air or under an inert atmosphere. Agitation is notnecessary during the heat treatment.

The duration of this treatment may vary over a wide range of from 2 to24 hours, preferably from 4 hours to 24 hours.

Upon completion of this operation, a solid precipitate is recovered,which is then separated by conventional liquid/solid separation methods,i.e., filtration, decantation, centrifugation, and the like.

The product obtained is then dried at a temperature varying from roomtemperature to 120° C., and preferably from 60° C. to 100° C. Thisoperation may be carried out in air or under a reduced pressure, forexample, a pressure of from 1 mm (133.32 Pa) to 100 mm of mercury (13332.2 Pa). The duration of the drying step is also not critical.

According to the invention, the cerium (IV) compound is prepared in ayield by weight, expressed as CeO₂, in relation to the amount of CeO₂introduced via the solution of the cerium (IV) salt, varying from 75 to98%.

The compound of the invention prepared by the aforedescribed process iscapable of directly forming an aqueous dispersion of the cerium (IV)compound, hereinafter referred to as a "sol".

Thus, the present invention also features the aqueous sols formulatedfrom the cerium (IV) compound having the formula (I).

The present invention also provides a process for the preparation of anaqueous sol of a cerium (IV) compound, whereby the compound of cerium(IV) corresponding to the formula (I) is placed in suspension in water.

The nature of the water is not critical and its temperature is generallyambient temperature.

Preferably, the preparation of said sol is carried out under agitation.

In one embodiment of the present invention, the compound of cerium (IV)is present in the form of a colloidal dispersion in water, whichsignifies that said compound comprises particles of colloidaldimensions, but does not exclude the presence of Ce (IV) in the ionicform.

An aqueous sol of a cerium (IV) compound may be prepared according tothe invention in a concentration, expressed as CeO₂, of up to 1.5moles/liter and preferably varying from 0.2 to 1.5 moles/liter.

A sol wherein the dimensions of the colloids vary over a rather widerange may thus be obtained.

The size of the colloids is defined as the measurement of thehydrodynamic diameter of the colloids determined by the quasi-elasticdiffusion of light described by Michael L. McConnel in AnalyticalChemistry, 53, No. 8, 1007 A (1981), with said diameter varying from 100to 1000 Å.

The sols obtained according to this invention are stable in storage;there is no settling after several months in storage.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative. Insaid examples to follow, all parts and percentages are by weight.

EXAMPLE 1 (a) Preparation of a cerium (IV) compound having the formula(I)

Into a three-necked, 2 liter flask, equipped with a thermometer, anagitator, and a system for the introduction of reagents (metering pump),100 cm³ of a commercial 17.5 N acetic acid were introduced and mixed atroom temperature with 400 cm³ of ceric nitrate solution containing 1.23moles/liter of cerium (IV), 0.05 mole/liter of cerium (III) and having afree acidity of 0.66 N, obtained by electrolysis according to FrenchApplication No. 2,570,087.

To this solution, which was maintained under agitation at roomtemperature for 5 hours, 34 min, a 3.6 N ammonium solution was added ata rate of 100 cm³ per hour.

After this addition, the reaction mixture contained cerium (IV) in aconcentration, expressed as CeO₂, of 80 g/l, in a molar ratio OH⁻ /Ce(IV) of approximately 3.5 and a molar ratio of acetic acid/Ce (IV) ofapproximately 3.5.

In a second stage, the reaction mixture was subjected to a heattreatment. For this purpose, the reaction mixture obtained above wasplaced in an oven at 80° C.

After 24 hours, a yellowish white precipitate was recovered byfiltration through sintered glass (No. 3 porosity).

The resultant product was then dried in air, in an oven heated to 80°C., for approximately 24 hours.

109.2 g of a precipitate displaying an ignition weight loss of about 25%were recovered.

A yield of the precipitation of cerium equal to was determined.

Chemical analysis of the final product gave the following results (theratios are molar ratios):

    ______________________________________                                        (i)         Ce (III)/Ce (IV) =                                                                          0.02                                                (ii)        NO.sub.3 .sup.- /Ce (IV) =                                                                  0.44                                                (iii)       NH.sub.4 .sup.+ /Ce (IV) =                                                                  0.17                                                (iv)        Acetate/Ce (IV) =                                                                           0.29                                                ______________________________________                                    

Analysis by X-ray diffraction evidenced the product of the invention tobe a poorly crystallized material having a crystalline CeO₂ phase of thefluorine type.

(b) Preparation of an aqueous sol of the cerium (IV) compound of theinvention

45.86 g of the compound prepared in step (a) were added to distilledwater in an amount sufficient to provide a total volume of 200 cm³.

A sol having a clear appearance and a concentration in cerium (IV),expressed as CeO₂, of 172 g/l (1 mole/liter) was prepared.

Examination by quasi-elastic light diffusion evidenced the presence ofcolloids having a hydrodynamic diameter on the order of 300 Å.

It is noted that the sol obtained was stable in storage and exhibited nosettling after at least one year.

EXAMPLE 2 (a) The procedure of Example 1 was repeated, with thefollowing differences:

(i) 400 cm³ of a solution of ceric nitrate containing 1.23 moles/literof cerium (IV), 0.05 mole/liter of cerium (III) and having a freeacidity of 0.66 N, were used;

(ii) 400 cm³ of concentrated 17.5 N acetic acid, were used; and

(iii) 138 cm³ of an 11 N ammonium solution were used, comprising 118 cm³of added water.

Upon completion of the reaction, the reaction mixture contained cerium(IV) in a concentration, expressed as CeO₂, equal to 80 g/l, a molarratio OH⁻ /Ce (IV) of approximately 2.5 and a molar acetic acid/Ce (IV)ratio of about 14.

In a second stage, the reaction mixture was subjected to a heattreatment in an oven at 100° C. for 24 hours.

After drying in air, 68.30 g of a product having the following chemicalanalysis were recovered:

    ______________________________________                                        CeO.sub.2 =           70%                                                     Molar ratio, acetate/Ce (IV) =                                                                      0.67                                                    ______________________________________                                    

(b) A sol as in Example 1 was prepared by the addition of 49.14 g of theproduct obtained above to distilled water in an amount sufficient toprovide a volume of 200 cm³.

A sol having a cerium (IV) concentration, expressed as CeO₂ of 172 g/l,was obtained.

Examination by quasi-elastic light diffusion carried out on an aliquotfraction diluted to 0.35 mole/liter of cerium (IV) showed the presenceof colloids having a hydrodynamic diameter of approximately 150 Å.

EXAMPLE 3

(a) The procedure of Example 1 was again repeated, with the followingdifferences:

(i) 450 cm³ of a ceric nitrate solution were used, containing 1.23moles/liter of cerium (IV), 0.05 mole/liter cerium (III) and having afree acidity of 0.66 N;

(ii) 50 cm³ of concentrated commercial acetic acid of approximately 17.5N were used; and

(iii) 693 cm³ of a 3.14 N ammonium solution were used.

Upon completion of the reaction, the reaction mixture contained cerium(IV) in a concentration, expressed as CeO₂, equal to 80 g/l, a molarratio OH⁻ /Ce (IV) of approximately 3.5 and a molar ratio, aceticacid/Ce (IV), of about 1.55.

In a second stage, the reaction mixture was subjected to a heattreatment in an oven at 100° C. for 24 hours.

After drying in air, 122 g of a product containing 70.5% CeO₂ wereobtained.

(b) A sol was prepared as in Example 1 by the addition of 48.79 g of theproduct obtained above to distilled water, in an amount sufficient toprovide a total volume of 200 cm³.

A sol having a cerium (IV) concentration, expressed as CeO₂, of 172 g/l,was obtained.

Examination by quasi-elastic light diffusion carried out on an aliquotfraction showed the presence of colloids having a hydrodynamic diameterof approximately 200 Å.

EXAMPLE 4

(a) The procedure of Example I was again repeated, but with thefollowing differences:

(i) 500 cm³ of a ceric nitrate solution were used, containing 1.54moles/liter of cerium (IV), 0.08 mole/liter cerium (III) and having afree acidity of 0.415 N;

(ii) 44 cm³ concentrated 17.5 N acetic acid were used; and

(iii) 1111.5 cm³ of a 2.61 N ammonium solution were used.

The reaction mixture contained cerium (IV) in a concentration, expressedas CeO₂, of 80 g/l, a molar ratio OH⁻ /Ce (IV) of approximately 3.5 anda molar ratio acetic acid/Ce (IV) equal to 1.

In a second stage, the reaction mixture was subjected to a heattreatment. For this purpose, the reaction mixture obtained above wasplaced in an oven heated to a temperature of 100° C.

After 12 hours, a precipitate was recovered by filtration on sinteredglass (No. 4 porosity).

The product obtained was then subjected to drying in air, carried out inan oven at 80° C. for approximately 24 hours.

161.3 g of a product having the following chemical analysis wererecovered:

    ______________________________________                                        (i)      CeO.sub.2 =          75%                                             (ii)     Molar ratio NO.sub.3 .sup.- /Ce (IV) =                                                             0.53                                            (iii)    Molar ratio NH.sub.4 .sup.+ /Ce (IV) =                                                             0.26                                            (iv)     Molar ratio acetic acid/Ce (IV) =                                                                  0.2                                             ______________________________________                                    

A cerium precipitation yield of 95% was determined.

X-ray diffraction analysis showed that the product obtained was poorlycrystallized and had a crystalline CeO₂ phase: the proportion ofcrystallization determined with respect to a control sample wasapproximately 35% and the size of the elementary crystallites was lessthan 30 Å.

(b) 45.86 g of the compound prepared according to step (a) were added todistilled water in an amount sufficient to provide a volume of 200 cm³.

A sol having a clear appearance and a cerium (IV) concentration,expressed as CeO₂, of 172 g/l (1 mole/liter), was obtained.

Examination by quasi-elastic light diffusion showed the presence ofcolloids having a hydrodynamic diameter on the order of 140 Å.

It was noted that the sol obtained had good stability in storage andevidenced no settling over time.

EXAMPLE 5

The procedure of Example 1 was again repeated, but with the followingdifferences:

(i) 500 cm³ of a ceric nitrate solution were used, containing 1.54moles/liter of cerium (IV), 0.08 mole/liter cerium (III) and having afree acidity of 0.415 N;

(ii) 57 cm³ concentrated, 99% commercial propionic acid (d=0.99 to 1.00)were used; and

(iii) 1098 cm³ of a 1.59 N ammonium solution were used.

Upon completion of the reaction, the mixture contained cerium (IV) in aconcentration, expressed as CeO₂, equal to 80 g/l, a molar ratio OH⁻ /Ce(IV) of approximately 2 and a molar ratio propionic acid/Ce (IV) of 1.

In a second stage, the reaction mixture was subjected to a heattreatment in an oven at 80° C. for 24 hours.

After drying in air, 84.1 g of a product containing 75% CeO₂ wererecovered.

A precipitation yield in cerium equal to 47.6% was determined.

(b) A sol was prepared as in Example 1, by the addition of 45.86 g ofthe product obtained above to distilled water in an amount sufficient toprovide a volume of 200 cm³.

A sol was obtained having a concentration in cerium (IV), expressed inCeO₂, of 172 g/l (1 mole/liter).

Examination by quasi-elastic light diffusion carried out on an aliquotfraction showed the presence of colloids having a hydrodynamic diameterof approximately 345 Å.

It was noted that the sol obtained had good stability in storage and didnot exhibit settling over time.

While this invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A cerium (IV) material having the general formula(I):

    Ce (M).sub.t (CH.sub.3 --CH.sub.2 ].sub.n COO.sup.-).sub.x (OH.sup.-).sub.y (NO3.sup.-).sub.z                                         (I)

wherein M is an alkali metal or a quaternary ammonium radical; t is avalue from the range of 0.1 to 0.3; n is 0 or 1; x is a value from therange of 0.1 to 0.7; y is a number such that y=4+t-x-z; and z is a valuefrom the range of 0.3 to 0.6.
 2. A sol comprising a suspension, inwater, of colloidal particulates of the cerium (IV) material as definedby claim
 1. 3. The aqueous sol as defined by claim 2, wherein theconcentration of the cerium (IV) material therein, expressed as CeO₂,ranges from 0.2 to 1.5 moles/liter.
 4. The aqueous sol as defined byclaim 3, wherein the hydrodynamic diameter of said colloidalparticulates ranges from 100 to 1000 Å.